Without specialized instruments such as altimeters and fuel gauges, pilots would not be able to fly airplanes, helicopters or any kind of aircraft well. When companies that own aircrafts need to buy new instruments or when aerospace companies need instruments for the aircrafts they are building, manufacturers of aircraft instruments and aircraft instrument distributors are turned to for purchases. Some aircraft instrument companies specialize in selling instruments for military aircraft while others may specialize in selling instruments for commercial airliners or private jets and private prop planes. There are also aircraft instrument manufacturers that specialize in selling helicopter instruments. Some aircraft instrument distributors also sell the instruments used by air traffic control personnel.